Neutron generators based on the deuteron-triton fusion reaction produce neutrons at an energy slightly over 14 MeV. At this energy, the probability of scattering from hydrogen is greatly reduced from that at epithermal energy, while the probability of inelastically scattering from heavier nuclei, like silicon, calcium, carbon and oxygen is greatly increased from that at lower energy. Consequently, scattering and energy loss are not dominated by hydrogen until the energy of the neutrons drops to approximately 6 MeV. By that time, the slowing down length (Lh) of the neutrons has already been heavily influenced by inelastic scattering. Thus, the transport of high energy neutrons is determined not only by hydrogen index, but also, to a large degree, by the density of inelastic scatterers and the inelastic scattering cross section of these nuclei. The slowing down of high energy neutrons relevant to inelastic scattering can be described by the (Lh) from 14 MeV to 1 MeV, which is similar to the conventional slowing down length (Ls), except that the latter extends down to thermal energy. In simple one-group infinite-media diffusion theory, the neutron flux at 1 MeV is proportional to
                                          ⅇ                          r                              L                h                                              r                ,                                        where r is the distance from the neutron source, and the inelastic gamma-ray production will have, approximately, this spatial dependence. Since the detection probability is high when the gamma-rays are produced close to the detector, the detection rate for detectors at distances rn and rf from the neutron source (where rn is the distance to the near detector and rf is the distance to the far detector) will be modulated by
                    ⅇ                              r            n                                L            h                                      r        n              ⁢                  ⁢          and        ⁢                  ⁢                  ⅇ                              r            f                                L            h                                      r        f              ,respectively. (1 MeV is approximately the lowest energy at which inelastic scattering is possible.) The ratio of detection rates will then vary as
            r      f              r      n        ⁢            ⅇ                        (                                    r              f                        -                          r              n                                )                          L          h                      .  For a small slowing down length increase ΔLh, the ratio of detection rates decreases at a rate proportional to ΔLh/Lh2. Since Lh varies little with (liquid filled) porosity, it is expected that the near:far inelastic gamma-ray ratio would also have little dependence on porosity. Note that this also implies that the near:far inelastic ratio would have no direct dependence on bulk formation density, since this varies with porosity. Of course, if the density of inelastic scatterers were to vary, then Lh and the near:far inelastic ratio would also vary. But this typically does not happen unless the lithology changes, such as from a sandstone to a shale.
The density of inelastic scatterers (e.g., carbon or oxygen) will decrease, however, if high density liquid is removed from pores and replaced with low density vapor. Such a change will cause Lh to increase and the near:far inelastic ratio to decrease. These changes can be used to measure the density of the pore fluid, given that the formation porosity is known independently.
In cased hole there are many nuclear techniques but most of them involve testing for the presence of hydrogen in the pores.
U.S. Pat. No. 6,805,195B2 to Vinegar, et al. discloses that a hydrocarbon containing formation may be treated using an in situ thermal process. A mixture of hydrocarbons, H2, and/or other formation fluids may be produced from the formation. Heat may be applied to the formation to raise a temperature of a portion of the formation to a pyrolysis temperature. After pyrolysis, the portion may be heated to a synthesis gas production temperature. A synthesis gas producing fluid may be introduced into the portion to generate synthesis gas. Synthesis gas may be produced from the formation in a batch manner or in a substantially continuous manner.
Likewise, U.S. Pat. No. 6,769,485B2 to Vinegar, et al. discloses that a coal formation may be treated using an in situ thermal process. Hydrocarbons, H2, and/or other formation fluids may be produced from the formation. Heat may be applied to the formation to raise a temperature of a portion of the formation to a synthesis gas production temperature. A synthesis gas producing fluid may be introduced into the formation to generate synthesis gas. Production wells may be operated at selected temperatures to obtain a desired synthesis gas composition.
Similarly, U.S. Pat. No. 6,761,216B2 to Vinegar, et al. discloses that a coal formation may be treated using an in situ thermal process. A mixture of hydrocarbons, H2, and/or other formation fluids may be produced from the formation. Heat may be applied to the formation to raise a temperature of a portion of the formation to a pyrolysis temperature. After pyrolysis, the portion may be heated to a synthesis gas production temperature. A synthesis gas producing fluid may be introduced into the portion to generate synthesis gas. Synthesis gas may be produced from the formation in a batch manner or in a substantially continuous manner.
In addition, U.S. Pat. No. 6,732,796B2 to Vinegar, et al., discloses that a hydrocarbon containing formation may be treated using an in situ thermal process. A mixture of hydrocarbons, H2, and/or other formation fluids may be produced from the formation. Heat may be applied to the formation to raise a temperature of a portion of the formation to a pyrolysis temperature. A portion of a formation may be heated from a plurality of heat sources to a temperature sufficient to allow generation of a first synthesis gas having a low H2 to CO ratio. A second portion of a formation may generate synthesis gas having a H2 to CO ratio greater than the first synthesis gas. A portion of the first synthesis gas may be blended with a portion of the second synthesis gas to produce a blend synthesis gas having a desired H2 to CO ratio.